Process for decarburizing metals



Patented Sept. 24, 1940 UNITED STATES PATENT OFFICE PROCESS FOR. DECARBURIZING METALS No Drawing. Application November 17, 1938, Serial No. 240,955

4 Claims. (01. 75-10) This invention relates to the production of metals and alloys containing only small amounts of carbon.

For the sake of conciseness, the invention will 5 be described with particular reference to the production of low carbon ferrochromium, but it will be apparent that the invention is applicable to the manufacture of metals and alloys including chromium, zirconium, vanadium, columbium, tantalum, molybdenum, tungsten, or manganese, and mixtures of two or more of such metals, with or without other metals such as iron.

In commerce, ferrochromium is classified according to its carbon content. Thus, ferrochromium containing more than 4% carbon is called high carbon ferrochromium and ferrochromium containing less than 2% carbon is called low carbon ferrochromium. High carbon ferrochromium is relatively easy and cheap to manufacture, but the production by direct methods of low carbon ferrochromium in which the ratio of chromium to carbon is high is much more difficult and expensive. There is a steadily growing demand for such low carbon ferrochromium which renders desirable the development of a simple and economical process for its production. One expedient which has been suggested is the decarburization of high carbon ferrochromium.

Many attempts have been made to provide economical methods of decarburizing high carbon ferrochromium, including smelting with chromium ore, treating a molten bath of ferrochromium with a blast of air, oxygen-enriched air, pure oxygen, or other gases. It has also been proposed to use hydrogen and steam in like manner and, in the manufacture of rustless iron, it has been proposed to treat the molten alloy with a blast of hydrogen. A disadvantage common to all of the methods heretofore proposed is that they involve an extensive loss of chromium, usually by oxidation, or the carbon content is not lowered by any considerable amount.

It is important that the chromium to carbon ratio be high because low carbon ferrochromium is used principally to make low carbon chromium steels in the manufacture of which it is desired to add to steel a large amount of chromium and as little carbon as possible. Thus, in decarburizing ferrochromium it is essential that the carbon content be lowered at a much faster rate than the chromium content is lowered.

It is an object of the presentinventionto provide a novel process for the production of metals having only a relatively small percentage of carbon. Another object is a process for decarburizing high carbon ferrochromium that avoids the disadvantages of previously proposed methods. A further object is to provide a method of raising the ratio of chromium to carbon in a ferrochromium alloy to 100 or more. 5

The invention is based on my discovery that super-heated molten ferrous metals may be readily decarburized by contact with hydrogen if the hydrogen is preheated to a very high temperature, and is a method that comprises treating a superheated molten bath of metal, having an undesirably high percentage of carbon, with hydrogen which is at a temperature substantially above the melting point of the said metal. The hydrogen may be blown upon the surface of the bath or 5 introduced into the bath in a manner analogous to that used in steel making converters, the surface of the bath preferably being free from substantial quantities of slag. If any small quantities of slag are present, e. g., such as may be adventi- 20 tiously formed from the furnace lining, such slag is preferably of a type adapted to be readily dispersed or volatilized by the arc, as is the case with, for example, high magnesia slags.

The beneficial results of the method of the in- 25 vention are enhanced if the molten metal is superheated by several hundred degrees centigrade.

It is essential that the hydrogen employed in the process of the invention be heated to a temperature substantially above the melting point of the 30 metal being treated before it is introduced to the bath. If the temperature of the hydrogen is lower,

a localized chilling effect is exerted on the bath, and the rate of decarburization is unduly slow. Such localized chilling effect is considerable, be- 35 cause of the high specific heat of hydrogen.

A preferred method of heating the hydrogen to the desired temperature is to subject it to the influence of an electric arc, as by passing it through or about the arc. The arc may be maintained be- 40 tween graphite electrodes or between such electrodes and the bath of molten metal. If, desired, the are used to heat the hydrogen may also be used to melt the metal, or to keep it molten, 0! both. 45

By the method of the invention, the carbon content of ferrochromium may be rapidly reduced to a very low value without any great oxidation of chromium. For instance, in one test embodying the invention, the carbon content of a ferrochro- 50 mium was reduced fromabout 1.8% to 0.064%, the chromium content being reduced from 72.14% to 71.43%. The ratio of chromium to carbon was raised from 40.1 to 111.6. In another test, the carbon content of a ferrochromium was reduced in forty minutes from 1.65% to 0.57% while the chromium content dropped from 71.47% to 69.8%. The ratio of chromium to carbon was raised from 43.3 to 122.5. 1 p

, The decarburization may be carried out in two stages, the first of which may be an oxidizing treatment and the second of which is a treatment with heated hydrogen. As an example of a two stage decarburization of ferrochromium, in the initial decarburization step, the carbon content of the ferrochromium is lowered from more than 4% to between 0.15% and 2% suitably by subjecting the molten bath of ferrochromium to the action of a blast of oxygen-enriched air, and such step is followed by a second decarburization step in which the bath is treated with heated hydrogen until the carbon content of the bath is less than 0.1%.

While certain specific examples of the invention have been described in connection with the production of low carbon ferrochromium, the

Q method of the invention is not limited to those may be carried out solely by blowing a blast of highly heated hydrogen onto the molten bath, or it may be accomplished in two stages, the first of which may be any known method of decarburization and the second of which is the hot hydrogen treatment of the invention.

I claim: 7

1. Method of decarburizing carbon-containing metal which comprises forming a highly superheated moltn bath of such metal; maintaining at least a' substantial part of the top surface of the bath substantially free from slag; maintaining an atmosphere of hydrogen over said bath and in contact with said top surface; and maintaining an electric arc in said atmosphere adjacent the bath to heat said hydrogen to a temperature above the melting point of said metal.

2. Method of raising the ratio of chromium to carbon of a carbon-containing iron-chromium alloy which comprises maintaining a highly superheated molten bath of such alloy in contact with a magnesia slag forming material; maintaining a substantially slag-free top surface on said bath; maintaining a hydrogen atmosphere over said bath; and heatingsaid bath and said hydrogen by an electric are maintained in said atmosphere between the bath and a carbon electrode.

3. Method as defined in claim 1, in which the metal is an iron-manganese alloy.

4. Method as defined in claim 1, in which the metal is an iron-chromiiim alloy.

AUGUSTUS B. KINZEL. 

